Systems for surface measurement are generally well known and have found application through industry and science. For example, jet engine turbine blades have many intricate features, including a multitude of coolant hole patterns and severe surface contours. Current inspection approaches for precision measurement of surface features are embodied in commercial coordinate measurement machines (CMMs). These systems use contact probe devices on precision translating mechanisms to measure a limited number of points on the part being inspected. In general, the part is stationary and the probe moves. Great care is taken to maintain accurate knowledge of the probe position. The inspection process is relatively slow because it is a sequential point-by-point process. The accuracy of the known and relatively fast ADAM machine is approximately 2 mils for a surface point size of approximately 0.0625 inches square. The much slower CMM units achieve about a 0.5 rail accuracy for a single point. As the complexity of the parts and the amount of time required for inspection increases, these approaches will become more and more limiting to future production processes.
Optical metrology techniques, in general, offer a way to increase the speed of the inspection process. This is because they are inherently 1- or 2-dimensional in nature, permitting an entire line segment or an area patch on a blade to be observed at once. With this observational speed, it becomes possible to determine the, entire surface contour, permitting a more thorough inspection than a limited contact probe or other sequential point-by-point measurement system would support. Until now, however, the available optical systems-have not had the accuracy and ruggedness to meet production inspection requirements.
One optical technique that is well suited to this application is Moire Interferometry. A Moire pattern is an intensity pattern (fringes) formed from the interference or light blocking when gratings, screens, or regularly spaced patterns are superimposed on one another. The interference fringes are most noticeable when the frequencies of the two interfering patterns are nearly the same. Common examples would be the shimmering phenomenon encountered when viewing through two separated screen windows or the shimmering pinstripe suit as seen on a television screen. When the spatial frequency of the pinstripes is closely matched to the pixel frequency of the TV camera, a Moire pattern appears.
Moire contouring techniques have been used for the accurate contouring of fairly elaborate surfaces. However, this technique has several fundamental drawbacks: 1) The geometry of the viewing system cannot allow for the measurement of large slopes; 2) the size of the grid and the viewing angle place stringent constraints on the size of the object; and 3) the mixing efficiency depends on the camera's ability to focus both the object and the grid at the same time. Moire systems have not been applied as high-throughput metrology instruments due to the tight tolerance required to register the image of the part with the reference grid.
It would be advantageous to have an optical metrologic system using Moire interferometry that was not limited by optical registration or depth of field constraints and yet would provide quick and accurate measurement of an object's surface contours. The present system is drawn towards such an invention.